1. Field of the Invention
This invention relates generally to devices which combine high frequency electrical energy from a plurality of microwave sources and, more particularly, to a power combiner which combines microwave power from the resonant cavities of a plurality of axially adjacent modular units, each of the modular units receiving power into a central resonant cavity from a number of circumferentially disposed microwave sources.
2. Description of the Prior Art
With the advent of advanced types of electromagnetic systems operating in the microwave frequency ranges, many different techniques have been employed to introduce semiconductor technology with its high reliability, small size, and low voltage characteristics to the generation of microwave power. Most present day power combining solid state apparatus involves the use of microwave diodes employed in either an amplifier or an oscillator circuit which operate at a very high frequency. A final output stage will typically have a number of such circuits, and the outputs from all of the circuits are connected in parallel to provide the desired level of output power. One technique for electrically coupling such circuits in parallel is to position these circuits around the periphery of a central cavity such that electromagnetic energy from each circuit propagates into the cavity to resonate with increased amplitude. Since spurious frequency generation and multiple moding of the cavity increases as a function of the cavity size, it is desirable to employ as small a cavity as possible to minimize mismoding and to optimize efficiency. However, a high output power requirement means a large number of diode circuits and thus considerations, such as the size of the individual diodes, the precise configuration of the associated waveguides, type of coupling to the cavity, and operating frequency are significant factors governing the overall physical size of the power combiner. Another factor to be considered is the relatively low output-to-input power ratio of the diodes operating in the microwave frequency range which has meant that power amplification most often involves cascading of stages, the later stages having more and more discrete individual microwave circuits. Accordingly, many different approaches to microwave power combining have been advanced to alleviate the inherent bulk and coupling problems associated with the large numbers of microwave circuits employed in the final amplifier stages.
One prior art technique utilizes a circuit approach towards power combining and is known as the "tree" technique. In this combining technique individual power sources at the first power level are grouped in pairs and have their outputs combined via waveguide to a second and higher power level. Similarly, at this second level inputs are again grouped in pairs and are combined via waveguides to form a higher power level with one-half of the original number of output leads. This technique of coupling the waveguides in parallel is repeated at successive power levels until a single waveguide output emerges at the last and highest power level. While in theory there is almost no limit to the number of microwave sources whose outputs can be combined in this manner, the practical limitations imposed by the overall size and cost of such a combiner make this approach undesirable.
Another technique for combining microwave power is to use a combiner with a separate input and output port having an elliptical cavity and a plurality of individual diode circuits positioned around the perimeter of the cavity. U.S. Pat. No. 3,873,935 issued Mar. 25, 1975 to H. Oltman, describes one such device and it shows a housing employing a plurality of elliptical cavities stacked in an axially adjacent configuration. Each cavity is shaped essentially as a right elliptical cylinder and receives an input microwave signal at the input focus. Microwave energy radiates radially outward from the input focus to the diode circuits around the periphery of the elliptical cavity. After amplification by the diode oscillator circuits, the microwave energy is reflected radially inwardly to the other focal point of the cavity which is connected to the output transmission line. A disadvantage of this elliptical structure is that the inherent noncircular configuration means that some of the diode locations are not symmetrically situated with respect to the cavity focal points and this causes interference patterns as microwave energy propagates to and from the peripheral locations. As a result, the electromagnetic fields arriving at the output focal point have a tendency to be slightly out of phase and therefore not totally additive, thereby reducing power combiner efficiency. Another disadvantage of the elliptical cavity concept is that both the individual diode circuits in each elliptical cavity do not operate under identical conditions of impedance and power level and this increases the likelihood of multimoding which also decreases unit efficiency.
Still another prior art approach to microwave power combining utilizes a plurality of oscillator circuits, each having a diode positioned at precisely spaced apart positions along the walls of a rectangular shaped cavity resonator. In U.S. Pat. No. 3,628,171 issued Dec. 14, 1971 to K. Kurokawa for MICROWAVE POWER COMBINING OSCILLATOR CIRCUITS microwave power from each individual diode circuit is radiated into the common central cavity. The spacing of the diodes along the cavity sidewall is particularly critical and the distance separating adjacent diodes must be within close tolerances to avoid multi-mode operation. Furthermore, the rectangular combining cavity is inherently large and bulky and it must be rather long to achieve acceptable microwave power levels since only two diode circuits can be accommodated at each station along the cavity spaced by multiples of one-half waveguide wavelength.
Yet another approach is described in U.S. Pat. No. 3,931,587 issued Jan. 6, 1976 to Harp et al. for MICROWAVE POWER ACCUMULATOR in which microwave power is combined in a center cylindrical cavity from a plurality of diode oscillator positioned around the cavity periphery. Power flow is directed from the diodes inwardly where it is combined in the central resonant cavity and is transmitted out of the unit by the single port. A disadvantage of this particular microwave power combining unit is that in order to operate the combining cavity at the most desirable mode for minimizing spurious oscillations, there is a limited number of circuits which can be disposed around the cavity periphery. Accordingly, to increase the power output of this accumulator, more diodes must be used and the size of the cavity will necessarily be increased to accommodate the additional oscillator circuits around its perimeter. Of course, with increased diameter the cavity will be designed to resonate at a higher mode increasing the possibility of mismoding and loss in efficiency.